Sheet conveying device and image forming apparatus including same

ABSTRACT

A sheet conveying device includes a conveying roller pair, a correction unit, and a control unit. The correction unit includes a correction roller pair and a roller holder. The control unit moves the roller holder from a reference position, and returns the roller holder to the reference position after the sheet is nipped by the correction roller pair, and after this sheet correction operation, the control unit releases the nip of the conveying roller pair. At least either a swing speed vh when returning the roller holder to the reference position, or a deflection amount of the sheet before the roller holder is returned to the reference position, is adjusted so that tension is not applied to the sheet between the conveying roller pair and the correction roller pair.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2019-21267 filed Feb. 8, 2019, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a sheet conveying device that conveys a sheet-like recording medium and is mounted in an image forming apparatus such as a facsimile machine, a copier, or a printer, and to an image forming apparatus including the sheet conveying device.

Image forming apparatuses including a facsimile machine, a copier, and a printer are configured to record an image on a sheet-like recording medium such as paper, fabric, or an OHP sheet. Such image forming apparatuses can be classified into an electrophotographic type, an inkjet type, and the like, in accordance with a recording method.

When printing on a sheet using the image forming apparatus, if the sheet is skewed with respect to a conveying direction, or if a positional shift occurs in a direction perpendicular to the conveying direction (in a sheet width direction), a print position for each sheet is shifted.

In a conventional image forming apparatus, a front end of the sheet is made to abut a nip of a registration roller pair in stopped state so that the sheet is deflected, and the front end of the sheet is set along the registration roller pair so that the skew of the sheet is corrected. However, this method has a problem that a positional shift in the sheet width direction cannot be corrected. Further, there is also a problem that collision noise occurs when the front end of the sheet is made to abut the registration roller pair.

Accordingly, there is proposed a method for correcting both skew and positional shift in the width direction of the sheet without using a registration roller pair. For instance, there is known a sheet conveying device including a skew correction roller pair that conveys the sheet, a skew detection sensor that detects skew of the sheet, and a sheet end position detection sensor that detects a side position of the sheet, in which the skew correction roller pair is inclined on the basis of a sheet inclination detected by the skew detection sensor and a side position of the sheet detected by the sheet end position detection sensor.

SUMMARY

A sheet conveying device according to one aspect of the present disclosure includes a conveying roller pair, an edge detection sensor, a skew detection sensor, a correction unit, and a control unit. The conveying roller pair conveys a sheet at a first speed v1. The edge detection sensor is disposed on a downstream side of the conveying roller pair in the conveying direction, so as to detect an edge position of the sheet in the width direction perpendicular to the conveying direction. The skew detection sensor is disposed on the downstream side of the conveying roller pair in the conveying direction, so as to detect skew of the sheet. The correction unit is disposed on the downstream side of the edge detection sensor and the skew detection sensor in the conveying direction, so as to correct skew and shift in the width direction of the sheet while conveying the sheet. The correction unit includes a correction roller pair that nips the sheet so as to convey the sheet at a second speed v2 slower than the first speed v1 and is capable of releasing its nip, a roller holder that supports the correction roller pair in a swingable manner in the conveying direction about a swing fulcrum on one end side in the sheet width direction and in a reciprocatable manner in the width direction. a skew correction motor that drives the roller holder to swing in the conveying direction, and a shift correction motor that drives the roller holder to move in the sheet width direction. The control unit controls the conveying roller pair and the correction unit basis on the detection results by the edge detection sensor and the skew detection sensor. The control unit performs a sheet correction operation which inclines the roller holder from a reference position by a predetermined angle on the basis of a detection result by the skew detection sensor before the front end of the sheet reaches the correction unit, returns the roller holder to the reference position after the sheet is nipped by the correction roller pair, and moves the roller holder in the sheet width direction basis on the detection result by the edge detection sensor. After this sheet correction operation, the control unit releases the nip of the conveying roller pair. At least either a swing speed vh when returning the roller holder to the reference position, or a deflection amount of the sheet before the roller holder is returned to the reference position, is adjusted so that tension is not applied to the sheet between the conveying roller pair and the correction roller pair.

Other objects of the present disclosure and specific advantages obtained by the present disclosure will become more apparent from the description of the embodiment given below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view illustrating a schematic structure of the image forming apparatus including a sheet conveying device of the present disclosure.

FIG. 2 is a plan view of the correction unit viewed from above.

FIG. 3 is a side view of the correction unit viewed from an upstream side in a sheet conveying direction.

FIG. 4 is a plan view of the sheet conveying device viewed from above according to one embodiment of the present disclosure.

FIG. 6 is a front view of an edge detection sensor viewed from the upstream side in the sheet conveying direction, showing a state in which a sheet conveying position is a center position.

FIG. 6 is a front view of the edge detection sensor viewed from the upstream side in the sheet conveying direction, showing a state in which the sheet conveying position is shifted from the center position toward one side in the width direction.

FIG. 7 is a block diagram showing control paths of the image forming apparatus of this embodiment.

FIG. 8 is a flowchart showing one example of lateral shift and skew correction control in the image forming apparatus of this embodiment.

FIG. 9 is a plan view showing a state in which a roller holder and a carriage of the correction unit are moved from a reference position when the paper sheet is skewed toward the right in the conveying direction.

FIG. 10 is a plan view showing a state in which a front edge of the paper sheet has reached a registration sensor after the state of FIG. 9.

FIG. 11 is a plan view showing a state in which the roller holder and the carriage of the correction unit are moved from the reference position when the paper sheet is skewed toward the left in the conveying direction.

FIG. 12 is a plan view showing a state in which the roller holder and the carriage of the correction unit are moved from the reference position, and a nip pressure of the first conveying roller pair is released.

FIG. 13 is a plan view showing a state in which the paper sheet has reached a second conveying roller pair after the state of FIG. 12.

DETAILED DESCRIPTION

Now, an embodiment of the present disclosure is described with reference to the drawings. FIG. 1 is a side cross-sectional view illustrating an internal structure of an image forming apparatus 100 including a sheet conveying device 31 of the present disclosure. In the image forming apparatus (such as a monochrome printer) 100, an image forming unit P is disposed, which forms a monochrome image by steps of charging, exposing, developing, and transferring. The image forming unit P includes a charging unit 2, an exposure unit (such as a laser scanning unit) 3, a developing unit 4, a transfer roller 5, a cleaning device 6, and a charge elimination device (not shown), which are disposed around a photosensitive drum 1 in a rotation direction thereof (in a clockwise direction in FIG. 1).

When performing image forming operation, the photosensitive drum 1 rotating in the clockwise direction is uniformly charged by the charging unit 2. Next, a laser beam from the exposure unit 3 based on document image data forms an electrostatic latent image on the photosensitive drum 1. Next, the developing unit 4 attaches developer (hereinafter referred to as toner) to the electrostatic latent image so that a toner image is formed.

The toner is supplied to the developing unit 4 from a toner container 8. Note that the image data is sent from a personal computer or the like. Further, a charge elimination device (not shown), which eliminates charge remaining on the surface of the photosensitive drum 1, is disposed on a downstream side of the cleaning device 6.

Toward the photosensitive drum 1 with the toner image formed as described above, a paper sheet (sheet) S is conveyed from a sheet feed cassette 10 or a manual sheet feed tray 11 by a sheet feed roller 12 a or 12 b along a sheet conveying path 13 via a first conveying roller pair (conveying roller pair) 14 and a second conveying roller pair 15 to the image forming unit P. Then, the toner image formed on the surface of the photosensitive drum 1 is transferred to the paper sheet S by the transfer roller 5.

The paper sheet S with the transferred toner image is separated from the photosensitive drum 1 and is conveyed to a fixing device 7 so that the toner image is fixed. The paper sheet S after passing through the fixing device 7 is conveyed to the upper part of the apparatus along a sheet conveying path 16, and is discharged onto a discharge tray 18 by a discharge roller pair 17 as it is (or after being branched to a reverse conveying path 20 from a branching unit 19 of the sheet conveying path 16 and after printing on both sides).

A correction unit 30 is disposed between the first conveying roller pair 14 and the second conveying roller pair 15. The correction unit 30 corrects skew and positional shift in the sheet width direction of the paper sheet S before sending out the paper sheet S toward the image forming unit P.

FIG. 2 is a plan view of the correction unit 30 viewed from above, and FIG. 3 is a side view of the correction unit 30 viewed from an upstream side in the sheet conveying direction (from the lower side in FIG. 2). The correction unit 30 includes correction roller pairs 50, a roller holder 51, a carriage 53, a roller drive motor 55, a skew correction motor 57, and a shift correction motor 59.

A plurality of pairs (four pairs in this example) of the correction roller pairs 50 are disposed in the sheet width direction (XX' direction). Each correction roller pair 50 is constituted of a drive roller 50 a and a driven roller 50 b. The roller holder 51 supports a rotation shaft 52 of the drive rollers 50 a in a rotatable manner. A swing fulcrum 51 a is disposed at one end side of the roller holder 51 in the sheet width direction (the left end side in FIGS. 2 and 3), and the other end side (the right end side in FIGS. 2 and 3) can swing in the sheet conveying direction about the swing fulcrum 51 a with respect to the carriage 53. The carriage 53 is supported by frames 101 a and 101 b on the front side and the back side of the printer 100 in a movable manner in the sheet width direction.

The roller drive motor 55 is connected to the rotation shaft 52 via a plurality of gears, so as to rotate and stop the rotation shaft 52. The skew correction motor 57 is connected to a rack 51 b attached to a swing end of the roller holder 51, via a plurality of gears, so as to change an inclination angle of the roller holder 51 with respect to the sheet conveying direction (Y direction). The shift correction motor 59 is connected to a rack teeth (not shown) formed on an end edge of the carriage 53, so as to reciprocate the carriage 53 in the sheet width direction. As each of the roller drive motor 55, the skew correction motor 57, and the shift correction motor 59, a stepping motor is used, whose rotation direction and rotation amount (rotation angle) can be accurately controlled by pulse control.

The frame 101 a is provided with a holder position sensor 60 for detecting a reference position (home position) of the roller holder 51. The holder position sensor 60 is a photointerrupter (PI) sensor that has a detection region (not shown) including a light emitting unit and a light receiving unit. An end portion of the roller holder 51 on the frame 101 a side is provided with a light blocking plate 51 b, and a position at which the light blocking plate 51 b blocks the detection region of the holder position sensor 60 is detected as the reference position of the roller holder 51. Note that the reference position in this description is a position at which the roller holder 51 is in the middle in the sheet width direction, and the inclination angle thereof with respect to the sheet conveying direction is zero (perpendicular to the conveying direction).

FIG. 4 is a plan view of the sheet conveying device 31 viewed from above, which conveys the paper sheet S from the manual sheet feed tray 11 to the image forming unit P in the image forming apparatus 100. Note that the sheet conveying direction (arrow Y direction) is the left to right direction in FIG. 4, which is opposite to FIG. 1 (the right to left direction).

The sheet conveying device 31 includes the correction unit 30, the first conveying roller pair 14 disposed on the upstream side of the correction unit 30 in the sheet conveying direction, edge detection sensors 25 a and 25 b, skew detection sensors 29 a and 29 b, a registration sensor 33, and a control unit 90 (see FIG. 7). Further, a sheet feed motor 61 that drives the sheet feed roller 12 b, and conveying motors 63 a and 63 b that drive the first conveying roller pair 14 and the second conveying roller pair 15 are disposed.

FIGS. 5 and 6 are front views of the edge detection sensors 25 a and 25 b viewed from the upstream side in the sheet conveying direction. The edge detection sensor 25 a detects an edge position of the paper sheet S on one side (the left side in FIGS. 5 and 6) in the width direction. The edge detection sensor 25 b detects an edge position of the paper sheet S on the other side (the right side in FIGS. 5 and 6) in the width direction.

Each of the edge detection sensors 25 a and 25 b is a PI sensor that has a detection region 27 (hatching regions in FIGS. 5 and 6) including a light emitting unit 27 a and a light receiving unit 27 b. The edge detection sensors 25 a and 25 b are disposed on both end portions in the width direction so that their detection regions 27 are opposed to each other. A distance W between the detection regions 27 is substantially the same as the width direction size of the conveyed paper sheet S (297 mm in lateral A4 size).

FIG. 5 shows a state where a conveying position of the paper sheet S is a center position. In this case, both end portions of the paper sheet S in the width direction do not overlap the detection regions 27 of the edge detection sensors 25 a and 25 b, and hence received light signal levels of the edge detection sensors 25 a and 25 b are both high.

FIG. 6 shows a state where the conveying position of the paper sheet S is shifted from the center position toward one side in the width direction (toward the left side in FIGS. 5 and 6). When the paper sheet S is shifted from the state of FIG. 5 toward one side in the width direction (toward the left side), the end edge of the paper sheet S overlaps the detection region 27 of the edge detection sensor 25 a as shown in FIG. 6, and the received light signal level of the edge detection sensor 25 a becomes low. Similarly, when the paper sheet S is shifted toward the other side in the width direction (toward the right side), the received light signal level of the edge detection sensor 25 b becomes low. In this way, shift of the paper sheet S toward one side (the left side) or the other side (the right side) is detected.

Output signals from the edge detection sensors 25 a and 25 b are sent to the control unit 90 (see FIG, 7). On the basis of detection results, the control unit 90 sends a control signal to the shift correction motor 59 of the correction unit 30 so that the carriage 53 is moved in the sheet width direction. Thus, the position in the sheet width direction of the roller holder 51 supported by the carriage 53 is changed from the reference position so that lateral shift of the paper sheet S is corrected.

The skew detection sensors 29 a and 29 b are a reflection-type PI sensor including a light emitting unit and a light receiving unit on a conveying surface of one side of the sheet conveying path 13 (see FIG. 1). The skew detection sensors 29 a and 29 b calculate skew amount (skew angle) of the paper sheet S, using a front edge deviation amount Δd (=v×t) oaf the paper sheet S calculated on the basis of detection timing difference t of the front edge of the paper sheet S and a conveying speed v of the paper sheet S, and a distance D between the skew detection sensors 29 a and 29 b, from the equation θ=tan⁻¹(Δd/D). The calculated skew amount is sent to the control unit 90. On the basis of the skew amount, the control unit 90 sends a control signal to the skew correction motor 57 of the correction unit 30, so as to correct skew of the paper sheet S by changing inclination of the roller holder 51 from the reference position.

The registration sensor 33 detects the front edge of the paper sheet S that has passed the correction unit 30. On the basis of detection timing of the front edge of the paper sheet S sent from the registration sensor 33, the control unit 90 controls rotation speeds of the roller drive motor 55 and the conveying motors 63 a and 63 b, so as to convey the paper sheet S in synchronization with image forming timing in the image forming unit P.

FIG. 7 is a block diagram illustrating one example of control paths of the image forming apparatus 100 of this embodiment. Note that control paths of the entire image forming apparatus 100 are complicated because various controls of individual portions are performed when using the image forming apparatus 100. Therefore, a part of the control paths, which is necessary for performing the present disclosure, is mainly described. Further, description of parts that are already described is omitted.

An image input unit 70 is a receiving unit that receives image data sent from a personal computer or the like to the image forming apparatus 100. The image signal input from the image input unit 70 is converted into a digital signal and is sent to a temporary storage unit 94.

An operation unit 80 is provided with a liquid crystal display unit 81 and an LED 82 that indicates various types of states, so as to indicates a status of the image forming apparatus 100 and to display an image formation status and the number of printed copies. Various settings of the image forming apparatus 100 are made using a printer driver in the personal computer.

The control unit 90 includes at least a central processing unit (CPU) 91, a read only memory (ROM) 92 as a storage unit dedicated to reading, a random access memory (RAM) 93 as a storage unit capable of reading and writing, the temporary storage unit 94 that temporarily stores image data or the like, a counter 95, a plurality of (two in this example) interfaces (I/F) 96 for sending control signals to individual devices in the image forming apparatus 100 or receiving an input signal from the operation unit 80.

The ROM 92 stores a control program of the image forming apparatus 100, numerical values or the like necessary for control, and data or the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated on the way of controlling the image forming apparatus 100, and data or the like that are temporarily necessary for controlling the image forming apparatus 100.

The temporary storage unit 94 temporarily stores an image signal that is input from the image input unit 70 for receiving image data from the personal computer or the like, and is converted into a digital signal. The counter 95 counts and accumulates the number of printed sheets.

Further, the control unit 90 sends control signals to individual parts and devices in the image forming apparatus 100 from the CPU 91 via the I/F 96. Further, signals indicating states of the parts and devices, and input signals are sent from the parts and devices to the CPU 91 via the I/F 96. The parts and devices controlled by the control unit 90 include, for example, the image forming unit P, the fixing device 7, the correction unit 30, the edge detection sensors 25 a and 25 b, the skew detection sensors 29 a and 29 b, the registration sensor 33, the holder position sensor 60, the image input unit 70, the operation unit 80, and the like.

FIG. 8 is a flowchart showing one example of skew and lateral shift correction control in the image forming apparatus 100. Along the steps of FIG. 8 and with reference to FIGS. 1 to 7 and FIGS. 9 to 12 described later, as necessary, a procedure for correcting skew and lateral shift of the paper sheet S is described in detail. Note that the case where the paper sheet S is fed from the manual sheet feed tray 11 is described here, but the same is true for the case where the paper sheet S is fed from the sheet feed cassette 10.

When conveying of the paper sheet S from the manual sheet feed tray 11 is started (Step S1), the control unit 90 determines whether or not the edge detection sensors 25 a and 25 b and the skew detection sensors 29 a and 29 b have detected the front edge of the paper sheet S (Step S2). If the edge detection sensors 25 a and 25 b and the skew detection sensors 29 a and 29 b have detected the front edge of the paper sheet S (Yes in Step S2), the control unit 90 calculates a lateral shift amount of the paper sheet S on the basis of detection signals from the edge detection sensors 25 a and 25 b. Further, the control unit 90 calculates the skew amount from the detection timing difference of the front edge of the paper sheet S, the conveying speed of the paper sheet S, and the distance between the skew detection sensors 29 a and 29 b (Step S3).

Next, in accordance with the lateral shift amount and the skew amount of the paper sheet 5, the roller holder 51 of the correction unit 30 is moved from the reference position (Step S4). Specifically, the lateral shift amount and the skew amount calculated in Step S3 are converted into the numbers of motor pulses of the shift correction motor 59 and the skew correction motor 57 necessary for correcting the lateral shift and the skew of the paper sheet S. The converted numbers of motor pulses are sent from the control unit 90 to the shift correction motor 59 and the skew correction motor 57, and thus the shift correction motor 59 and the skew correction motor 57 are driven by the numbers of motor pulses so that the roller holder 51 is moved.

FIG. 9 is a plan view showing a state in which the roller holder 51 and the carriage 53 of the correction unit 30 are moved. In FIG. 9, the paper sheet S is skewed toward the right in the conveying direction (toward the lower side in FIG. 9). Therefore, the roller holder 51 is inclined by the skew angle in the clockwise direction from the reference position (broken line position in FIG. 9) so as to be parallel to the front edge of the paper sheet S.

Next, the control unit 90 determines whether or not the registration sensor 33 has detected the front edge of the paper sheet S (Step S5). FIG. 10 is a plan view showing the state where the front edge of the paper sheet S has reached the registration sensor 33 after the state of FIG. 9. Linear speeds of the first conveying roller pair 14, the correction roller pair 50, and the second conveying roller pair 15 are represented by v1, v2, and v3, and then they are set so that v1>v2>v3 holds. Therefore, a deflection (loop) is formed in the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50.

If the registration sensor 33 has detected the front edge of the paper sheet S (Yes in Step S5), the control unit 90 sends a control signal to the shift correction motor 59 and the skew correction motor 57 so that the roller holder 51 is returned to the reference position. Specifically, the control unit 90 rotates the shift correction motor 59 and the skew correction motor 57 in directions opposite to those in Step S4. Further, when light of the holder position sensor 60 is blocked by the light blocking plate 51 b, the control unit 90 stops the shift correction motor 59 and the skew correction motor 57. In this case, the skew correction motor 57 drives the roller holder 51 to return to the reference position at a swing speed vh slower than the linear speed v1 of the first conveying roller pair 14 (Step S6).

Note that the swing speed vh in this description means a moving speed of the roller holder 51 in a direction parallel to the conveying direction. Therefore, a moving speed of the roller holder 51 in a direction perpendicular to the conveying direction (in the sheet width direction) is not considered in the lateral shift correction. Further, although the roller holder 51 rotates (swings) in the skew correction, a rotation angle (swing angle) of the roller holder 51 is small, and hence the swing speed vh is regarded as a linear speed (moving speed in the conveying direction) in this description.

As illustrated in FIG. 9, if the paper sheet S is skewed toward the right in the conveying direction, the swing end (left end) of the roller holder 51 is swung from the reference position to the downstream side in the conveying direction. In this case, when returning the roller holder 51 to the reference position, the swing end of the roller holder 51 is swung to the upstream side in the conveying direction. Therefore, no pulling tension is applied to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50.

On the contrary, if the paper sheet S is skewed toward the left in the conveying direction, the swing end (left end) of the roller holder 51 is swung from the reference position to the upstream side in the conveying direction as illustrated in FIG. 11. In this case, when returning the roller holder 51 to the reference position, the swing end of the roller holder 51 is swung to the downstream side in the conveying direction. Therefore, when the roller holder 51 is returned to the reference position, if the swing speed vh is high and the deflection amount of the paper sheet S is small, a pulling tension is applied to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 along with the movement of the roller holder 51. As a result, the roller drive motor 55 driving the correction roller pair 50 or the conveying motor 63 a driving the first conveying roller pair 14 may be out of order, or a wrinkle or break may occur in the paper sheet S.

Therefore, in the sheet conveying device 31 of this embodiment, a relationship between the swing speed vh when returning the roller holder 51 to the reference position and the deflection amount of the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 before returning the roller holder 51 to the reference position is optimized, so as to avoid application of the pulling tension to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50.

For instance, if the swing speed vh satisfies the relationship vh≤v1−v2, the sum of the linear speed v2 of the correction roller pair 50 and the moving speed of the roller holder 51 in the conveying direction, i.e. v2+vh, is always lower than or equal to the conveying speed v1 of the first conveying roller pair 14. As a result, regardless of the deflection amount of the paper sheet S, application of the pulling tension to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 along with the movement of the roller holder 51 can be securely avoided. In other words, even if the deflection amount of the paper sheet S is zero, application of the pulling tension to the paper sheet S can be avoided.

Alternatively, if the deflection amount of the paper sheet S is larger than or equal to the maximum movement amount of the roller holder 51 in the conveying direction, regardless of the swing speed vh of the roller holder 51, tension force applied to the paper sheet S can be absorbed by the deflected part, and hence application of the pulling tension to the paper sheet S can be securely avoided. In other words, even if vh>v1−v2 holds, application of the pulling tension to the paper sheet S can be avoided. As a method for adjusting the deflection amount of the paper sheet S, the linear speed of the correction roller pair 50 should be decreased (or the correction roller pair 50 should be temporarily stopped), or the linear speed of the first conveying roller pair 14 should be increased so that a linear speed ratio of the correction roller pair 50 to the first conveying roller pair 14 is decreased.

Next, nip pressure of the first conveying roller pair 14 is released (Step S7). FIG. 12 is a plan view showing a state in which the roller holder 51 is moved to the reference position, and nip pressure of the first conveying roller pair 14 is released. As illustrated in FIG. 12, when the nip by the first conveying roller pair 14 is released at the rear end of the paper sheet S, the deflection of the paper sheet S is canceled so that the paper sheet S becomes parallel to the conveying direction and is conveyed at the center position in the sheet width direction.

Next, the first conveying roller pair 14 is made to contact again (Step S8) so as to nip the paper sheet S. FIG. 13 is a diagram showing a state in which the paper sheet S has reached the second conveying roller pair 15 after the state of FIG. 12. The control unit 90 sends control signals to the conveying motors 63 a and 63 b, and the roller drive motor 55, so as to control the conveying speed of the paper sheet S in accordance with image forming timing in the image forming unit P (Step S9).

After that, it is determined whether or not printing is finished (Step S10). If the printing is continuing (No in Step S10), the process returns to Step S1, and the same procedure is repeated. If the printing is finished (Yes in Step S10), the process is finished.

According to the control described above, both the lateral shift and the skew of the paper sheet S can be corrected at the same time using the correction unit 30. Therefore, compared with conventional skew correction using a registration roller pair, positioning accuracy of an image in the width direction of the paper sheet S can be improved.

Further, it is not required to stop the paper sheet S by the registration roller pair, and hence collision noise between the paper sheet S and the registration roller pair can be reduced. Further, when printing continuously, there is no restriction of an interval between paper sheets S (sheet interval) in consideration of the sheet stop time at the registration roller pair, and hence productivity (image forming efficiency) can be improved.

Further, by adjusting the swing speed vh when returning the roller holder 51 to the reference position and the deflection amount of the paper sheet 5, application of the pulling tension to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 can be avoided, and hence out of order of the roller drive motor 55 or the conveying motor 63 a, or a wrinkle or break of the paper sheet S can be suppressed.

In particular, by setting the swing speed vh of the roller holder 51 to satisfy vh≤v1−v2, or by setting the deflection amount of the paper sheet S to be larger than or equal to the maximum movement amount of the roller holder 51 in the conveying direction, application of the pulling tension to the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 can be securely avoided.

Further, by releasing the nip pressure of the first conveying roller pair 14 after lateral shift and skew of the paper sheet S is corrected by the correction unit 30, deflection of the paper sheet S between the first conveying roller pair 14 and the correction roller pair 50 can be canceled, and hence conveying attitude and conveying position of the paper sheet S can be smoothly corrected.

Other than that, the present disclosure is not limited to the embodiment described above, and various modifications can be made within the scope of the present disclosure without deviating from the spirit thereof. For instance, although PI sensors are used as the edge detection sensors 25 a and 25 b, the skew detection sensors 29 a and 29 b, the registration sensor 33, and the holder position sensor 60 in the embodiment described above, it is possible to adopt a method of combining an actuator and a PI sensor, or other sensor such as a contact image sensor (CIS) may be used.

Further, the embodiment described above exemplifies the electrophotographic image forming apparatus 100, in which an electrostatic latent image is formed on an image carrier such as a photosensitive drum using a laser beam, toner is attached to the electrostatic latent image so that a toner image is formed, then the toner image is transferred onto the paper sheet (recording medium), and the transferred toner before fixing is heated and pressed so that a permanent image is formed. However, the present disclosure is not limited to the electrophotographic image forming apparatus, but may be applied also to an inkjet type image forming apparatus, for example, in which an ink ejection nozzle of a line head or a serial head ejects ink onto the paper sheet S so that an image is formed.

The present disclosure can be applied to a sheet conveying device that convey a sheet-like recording medium. Applying the present disclosure, it is possible to provide a sheet conveying device that can accurately correct skew and positional shift in a width direction of a sheet with a simple structure and can avoid application of pulling tension to the sheet in the correction process, and an image forming apparatus including the sheet conveying device. 

What is claimed is:
 1. A sheet conveying device comprising: a conveying roller pair for conveying a sheet at a first speed v1; an edge detection sensor disposed on a downstream side of the conveying roller pair in a conveying direction so as to detect an edge position of the sheet in a width direction perpendicular to the conveying direction; a skew detection sensor disposed on the downstream side of the conveying roller pair in the conveying direction so as to detect skew of the sheet; a correction unit disposed on the downstream side of the edge detection sensor and the skew detection sensor in the conveying direction, the correction unit including a correction roller pair that nips the sheet conveyed by the conveying roller pair so as to convey the sheet at a second speed v2 slower than the first speed v1 and is capable releasing the nip, a roller holder that supports the correction roller pair in a swingable manner in the conveying direction about a swing fulcrum on one end side in the sheet width direction, and in a reciprocatable manner in the width direction, a skew correction motor that drives the roller holder to swing in the conveying direction, a shift correction motor that drives the roller holder to move in the sheet width direction, and a control unit controls the conveying roller pair and the correction unit basis on the detection results by the edge detection sensor and the skew detection sensor, wherein the control unit performs a sheet correction operation which inclines the roller holder from a reference position by a predetermined angle on the basis of a detection result by the skew detection sensor before the front end of the sheet reaches the correction unit, returns the roller holder to the reference position after the sheet is nipped by the correction roller pair, and moves the roller holder in the sheet width direction basis on the detection result by the edge detection sensor, and after this sheet correction operation, the control unit releases the nip of the conveying roller pair, and at least either a swing speed vh when returning the roller holder to the reference position, or a deflection amount of the sheet before the roller holder is returned to the reference position, is adjusted so that tension is not applied to the sheet between the conveying roller pair and the correction roller pair.
 2. The sheet conveying device according to claim 1, wherein the swing speed vh satisfies vh≤v1−v2.
 3. The sheet conveying device according to claim 1, wherein the deflection amount is larger than or equal to a maximum movement amount of the roller holder in the conveying direction.
 4. The sheet conveying device according to claim 1, further comprising a registration sensor disposed on the downstream side of the correction unit in the conveying direction so as to detect passing of the sheet, wherein the control unit returns the roller holder to the reference position at timing when the registration sensor detects the front end of the sheet.
 5. The sheet conveying device according to claim 1, wherein the skew correction motor and the shift correction motor are stepping motors capable of controlling rotation direction and rotation amount by pulse control.
 6. An image forming apparatus comprising: the sheet conveying device according to claim 1; and an image forming unit arranged to form an image on the sheet conveyed by the sheet conveying device. 